Glial cells are thought to derive embryologically from either myeloid cells of the hematopoietic system (microglia) or neuroepithelial progenitor cells (astroglia and oligodendrocytes). However, it is unclear whether the glia in adult brains free of disease or injury originate solely from cells present in the brain since the fetal stage of development, or if there is further input into such adult brains from cells originating outside the central nervous system (CNS).
Besides the cells of the vasculature, the brain comprises two general cell types: neurons and glial cells. Glial cells provide physiological support to neurons and repair neuronal damage due to injury or disease. Macroglia (astroglia and oligodendroglia) are generally considered to be derived from neuro-ectoderm and are believed to be developmentally distinct from microglia (1). However, the developmental origin of microglia remains debatable (2,3). The two major views are that they derive either from neuro-epithelial cells (4–6) or from hematopoietic cells (i.e., monocytes) (7,8). The extent to which cells outside the CNS contribute to the maintenance of microglia in adults remains debatable (compare (9) and (10)), and no such contribution to adult neurons or macroglia has been previously described.